1. Field of the Invention
The present invention relates generally to aircraft design, specifically to aircraft wing design, and more specifically to aircraft wing design which would significantly increase aircraft efficiency and obviate the necessity for aircraft adverse yaw controls.
2. Description of the Related Art
When Wilbur and Orville Wright test flew gliders in 1900 and 1901, they discovered a problem with the control of their gliders. When they attempted to put in roll control, the wing they would increase the lift on would move backwards. In other words, the aircraft would roll one way, but it would yaw the opposite way, causing the gliders to crash. This is called adverse yaw, yawing the opposite direction to the roll command to turn. In 1902, the Wrights solved this problem by adding a rudder. The Wrights were awarded a patent for this design in 1906.
Current aircraft design includes two methodologies to control the adverse yaw identified by these aircraft pioneers. The first is the tail/rudder developed by the Wright brothers and the second is create devices at the wingtips that allow the aircraft to manipulate drag at the wing tips (split elevons at the tip like the B-2 Spirit aircraft, for example).
The wing designs of current aircraft that employ these types of yaw control are based, in part, on a paper published by Ludwig Prandtl in 1920 (NACA Report No. 116) which describes the theory called the Lifting Line, which becomes a mathematical tool by which the calculation of a wings' performance was first set forth. Other theories exist, but are too cumbersome to use, or too simplistic to be of value. Prandtl's Lifting Line is the first tool that provides meaningful results for wings. In this paper, Prandtl also introduces the concept of the elliptical span load as being the minimum induced drag for a given lift and a given wingspan.
Shortly thereafter Max Munk, Prandtl's student, published a paper, in NACA Report No. 120, that also describes a stagger biplane solution (often referred to as the stagger biplane report). This report describes that the elliptical span load results in a constant downwash behind the wing, and that the induced drag along the span of the wing is approximately elliptical as well.
In 1932, Prandtl published a paper on the minimum induced drag of wings, “Uber Tragflugel Kleinsten Induzierten Widerstandes” [this translates as: On the Minimum Induced Drag of Wings] (Zeitschrift fur Flugtecknik and Motorluftschiffahrt, 28 XII 1932; Munchen, Deustchland). In this paper, Prandtl attempts to determine a span load that uses the same amount of structure and produces the same lift, but has less induced drag than the elliptical span load. Prandtl uses the structure as the constraint, along with the lift by enforcing the same integrated wing bending moment of the elliptical on a new span load. Prandtl shows that this new span load produces a downwash at the centerline, but the downwash decreases moving outboard and becomes an upwash at the wing tip. Prandtl proposes that the wing planform be used to create this new span load (Prandtl refers to this wing design as the “sharp tipped wing”) and that the new span load has 22% more span and 11% less induced drag than the elliptical span load, but the same lift and the same integrated wing bending moment (the same structure). Although the paper does not disclose this, it implies that the induced drag begins at the wing centerline, decreases moving outboard and becomes negative induced drag at the wing tips (negative induced drag is induced THRUST). Therefore, the span load contemplated is a bell shape, rather than elliptical.
In 1934, two teenage brothers, Walter and Reimar Horten, begin building a series of gliders that use Prandtl's proposed span load. Reimar Horten coins the term “bell shaped span load” for this shape. Over the next 20 years, they attempt to develop the idea. The Hortens never fully explain how to create the wings associated with the proposed bell shaped span load. Their work is documented in the book “Nurflugel” by Reimar Horten, Peter Selinger, and Jan Scott (H Weishaupt Verlag, 1993).
Robert T Jones of the NACA Ames Aeronautical Laboratory publishes a paper, NACA Technical Note 2249 “The Spanwise Distribution of Lift for Minimum Induced Drag of Wings Having a given Lift and a Given Bending Moment.” This problem solution is nearly identical to the one Prandtl had solved 18 years earlier, but Jones was unaware of Prandtl's solution. Jones' solution also produced a bell shaped span load, a similar distribution of downwash/upwash (with induced thrust at the wingtips), and a similar distribution of induced drag as Prandtl's 1932 solution. Jones solution uses 26% more span, has 17% less induced drag, the same lift and the same wing root bending moment as the elliptical span load it is derived from. Jones also proposed to use planform to produce this new span load.
Although some of this early research described a potential for reducing induced drag on an aircraft wing by creating a bell shaped span load, little serious design and development work resulted from these theoretical findings, partly due to the impracticality of using planform to produce such a span load.
Finally, one recent technique has been developed to use twist distribution along the wing in order to minimize induced drag by varying the washout (U.S. Pat. No. 6,970,773). However, this technique employs a linear twist that still results in an elliptical span load and, therefore, does not provide yaw control without a standard rudder.
Therefore, it is desired to provide a wing design that can create a bell shaped span load, thereby reducing drag on the wing, without relying solely on planform techniques and, in addition, create yaw control without the need of a plane rudder or tail.